I think it may be due to stressing the body with the fast causing over-stimulation via "fight or flight" hormones, or just mild dehydration due to ketosis and not drinking enough in the hot weather? Several people have reported 5:2 fasting causing this.

I check mine bid, I have a history of high systolic. It does go up until I eat something around 1:30 PM then it goes low -- strange indeed. In any case I will start drinking more liquids and Topo-Chico mineral water than ever. I noticed my Walmart has sold out of the imported sparkling water beverage in this Summer heat wave.What I opine, if you fast, get a meter and check BP two to three times daily. I have a prescription beta blocker to take if mine gets up and stays elevated which it hasn't yet. I also spot-check FBS and it has been below 100 upon rising and 80's during the day.